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Northwest Africa 2975 – 70.1 Grams

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Northwest Africa 2975 – 70.1 Grams Northwest Africa 2975 – 70.1 grams NWA2986 – 201 grams NWA2987 – 225 grams ? NWA4766 – 47 grams NWA4783 – 120 grams NWA4857 – 24 grams NWA4864 – 94 grams NWA4878 – 130 grams NWA4880 – 81.6 grams NWA4930 – 117.5 grams NWA5140 – 7.5 grams NWA5214 – 50.7 grams NWA5219 – 60 grams NWA5313 – 5.3 grams NWA5366 – 39.6 grams Enriched fine-grained shergottite (shower) ~1.6 kg Figure 1: Photograph of NWA 2975 taken by Mike Farmer. Cube is 1 cm. Introduction Abstracts by Wittke et al. (2006) and Bunch et al. (2008) describe a shower of small (~100 g each) bits and pieces of basaltitc shergottite (there may be over a hundred pieces). Many have a fresh fusion crust (partial), a basaltic texture and the characteristic glass pockets and thin black glass veins such as are seen in Zagami and other basaltic shergottites. These glass pockets will prove important (see sections on EETA79001 and Zagami). 1 The first specimen, NWA 2975, was originally purchased by Mike Farmer in Erfoud, Morocco, but the suspected strewnfield is thought to be in Algeria (Bunch et al., 2008). Since then additional members have been found, including NWA 2986, 2987, 4766, 4783, 4857, 4864, 4878, 4880, 4930, 5140, 5214, 5219, 5313, and 5366 (Figures 2 to 6). Figure 2: Interior of NWA 2975 showing vesicular glass pockets and veins. Photo by Mike Farmer. Petrography According to Wittke et al. (2006), NWA 2975 is a fresh, medium-grained, subophitic to granular hypabyssal basalt with intergrown prismatic pyroxene and plagioclase grains up to 3 mm long (Figure 7). The hand specimen also exhibits vesicular black glass veins up to 3 mm wide and glass pockets up to 6 mm (Figure 2). Accessory phases include ülvospinel, ilmenite (Figure 8), chlorapatite, merrillite, pyrrhotite, Si-Al-K-Na-rich glass and baddeleyite. Large ülvospinel grains contain melt inclusions. 2 Figure 3: Sawn surface of NWA 4766 (photo by S. Ralew). Figure 4a,b: Fusion crust on outer surface and sawn surface of NWA 4766 (photos by S. Ralew). Figure 5 (left): Photo of ablation surface of NWA 4880 (photo from “Hupe Collection”). Figure 6 (right): Exterior of NWA 2986 (photo by J. Farmer). 3 Figure 7a: Photomicrograph of thin section of NWA 2986 (J. Wittke and T. Bunch). Photo is 8 mm across. Figure 7b: False color image of combined X-ray Ca (green)-Fe (blue)-Al (red). The numbers shown are the Mg# number of the pyroxenes (from He et al., 2015). Figure 8: Petrographic features of phosphates and Fe-Ti oxides in NWA 2975. A) phosphate (Mer) and Si-rich melts intruded in grains Fe-rich pigeonite (Ferro-Pgn). B and C) Trapped melt inclusions or phosphate and Si-rich melts intruded in grains of Fe- Ti oxides. The melts exhibit marginal rims of Fe-rich pigeonite + merrillite surrounding cores of Si-Al-K-Na-rich glass in image (B). D) Ti-magnetite grains have ilmenite exsolution lamellae. Images and figure from He et al., 2015. 4 Mineralogical Mode for NWA2975 Wittke et al. 2006 Pyroxene 57.3 vol. % Plagioclase 38.3 (maskelynite) Opaques 2.7 Phosphate 1.7 Mineral Chemistry Pyroxenes: The pyroxenes in NWA 2975 are relatively iron rich with exsolution features (Figure 9). Both augite and pigeonite are present. The Mn/Fe ratio demonstrates samples are Martian. Figure 9: Pyroxene composition of NWA 2975 compared with that of some other martian shergottites (from Wittke et al., 2006). Maskelynite: Plagioclase in NWA2975 has entirely been converted to maskelynite An55 (Wittke et al. 2006). Glass: Glass pockets are vesicular. Chromite: Ülvospinel. Sulfide: Pyrrhotite. Phosphate: NWA 2975 and NWA 4864 have reported merrillite and apatite (Channon et al. 2011; Slaby et al., 2016, 2017; Shearer et al., 2015). The apatite studied by Slaby et al. (2016) is F-rich hydroxyl-apatite (Figure 10). Slaby et al. (2017) found a more diverse phosphate assemblage and proposed the crystallization of two generations of fluorapatite: apatite crystals crystallized from an evolved, differentiated, and degassed magma, whereas chlorapatite replaced ferromerrillite-merrillite and was not related to the mantle- derived shergottite magma. The relationship between merrillite and apatite indicates that 5 apatite is most probably a product of merrillite reacting with fluids. (Figure 11). Slaby et al. (2017) suggest that only the fluorapatite (F > Cl~OH) is a reliable source for assessing the degree of Martian mantle hydration. In addition, Ward et al. (2017) report REE and other trace element data for the apatite and merrillite from NWA 4864, showing its similarity to other shergottite merrillites (Figures 12 and 13) Ilmenite: Attached to ülvospinel. Figure 10: Cl-OH-F ternary diagram for apatite from NWA 2975 (from Slaby et al., 2016). Figure 11: Two examples of the textural relations between apatite and merrillite in NWA 2975 (from Slaby et al., 2017). 6 Figure 12: REE patterns for merrillite from NWA 4864 showing its similarity to other shergottite merrillites (from Ward et al., 2017). Figure 13: Trace element patterns for apatite and merrillite from NWA 4864 showing their similarity to each other and to those in other shergottites (from Ward et al., 2017). Table 1. Chemical composition of NWA 2975 and pairs. reference Weisberg (2986) Yang (5214) weight technique ICP-MS LA-ICP-MS SiO2 % 47.4 TiO2 0.87 Al2O3 6.50 7 FeO 20.54 MnO 0.563 MgO 8.42 CaO 12.39 Na2O 1.5 K2O 0.34 P2O5 1.45 S % 0.03 sum Sc ppm 73 V 236 320 Cr 646 832 Co 29 37 Ni 47 51 Cu 13.8 Zn 81 Ga 17.0 Ge 1.36 As ppb 350 Se 400 Rb ppm 17.7 Sr 63 Y 29 Zr 74 Nb 4.06 Mo - Ru ppb - Rh - Pd - Ag 7 Cd - In 69 Sn 404 Sb 14 Te - Cs 1135 Ba ppm 21 45.7 La 0.97 3.93 Ce 2.32 9.50 Pr 1.38 Nd 1.7 6.98 Sm 0.71 2.67 Eu 0.43 0.95 Gd 1.71 4.40 Tb 0.81 Dy 1.12 5.42 Ho 1.12 Er 3.13 Tm 0.43 Yb 0.85 2.67 Lu 0.12 0.38 Hf 1.07 2.23 Ta ppb 187 W ppb 451 8 Re ppb - Os ppb 0.86 Ir ppb 0.18 Pt ppb 0.7 Au ppb 0.6 Tl 38 Pb ppm 1.15 Bi ppb 29.0 Th ppb 602 U ppb 154 Whole-rock Composition Weisberg et al. (2010) reported a preliminary analysis of NWA 2986 (Table 1). NWA 5214 was analyzed for a more extensive list of elements which clearly show the evolved and enriched nature of this basaltic shergottite (Yang et al., 2015). The siderophile elements were used to help constrain the conditions of core-mantle differentiation in Mars, and the Ge contents have been used to understand magmatic degassing processes (Humayun et al., 2016; Yang et al. 2018). Sanborn and Wadhwa (2010) were able to determine that the minerals in NWA 2975 crystallized from an “enriched” magma. Wittke et al. (2010) reported a difference in composition of NWA 5718 and NWA 2975, indicating that NWA 5718 is not paired. The REE contents of the individual phases in NWA 2975 are nearly identical to those in Shergotty and Zagami (Figure 14), demonstrating the overall similarity in composition between all these enriched shergottites. 9 Figure 14: Representative REE concentrations of pyroxenes, maskelynite, and merrillite of NWA 2975. For comparison, the mineral compositions of Shergotty and Zagami are shown after Wadhwa et al. (1994) and McCoy et al. (1999). Radiogenic Isotopes Cassata and Borg (2016) report Ar-Ar data for NWA 2975 and calculate an age of 184 +/- 17 Ma when corrected for the trapped 36Ar component (Figure 15). This is in reasonable agreement with Borg et al. (2016) who measured Rb-Sr and Sm-Nd isotopes in NWA 4878 and demonstrate its young age (170 Ma; Borg et al., 2016), similar to other enriched basaltic shergottites. In addition, they demonstrated, together with analyses of many other martian meteorites, that the age of the sources region must be ~ 4.5 Ga (Figure 16). 10 Figure 15: 36Ar/40Ar versus 39Ar/40Ar plot for NWA 2975 showing the result of corrections due to 36Ar cosmogenic, which lead to an age of 184 +/- 17 Ma (from Cassata and Borg, 2016). Figure 16: A 146Sm-142Nd isochron plot used to determine the age of formation of the shergottite source regions. This isochron yields a 146Sm/147Sm slope of 0.001089 ± 43 (MSWD = 0.48) that corresponds to an age of 4503 ± 6 Ma. Diagram from Borg et al. (2016). Cosmogenic Isotopes Berezhnoy et al. (2008) determined that 10Be, 26Al and 53Mn activity of NWA 2975. They calculate a cosmic ray exposure age of 1.6 m.y. Cassata and Borg (2016) determined cosmic ray exposure age from Ar analyses of un-irradiated aliquots of maskelynite and obtained an age of 1.62 +/- 0.10 Ma. Other Isotopes Many stable isotope studies are aimed at deciphering the building blocks of Mars, such as Li, Mg, and Cl isotopes: Li - Lithium isotopes have been measured for NWA 4864, and have helped to define the Li isotopic composition of the martian mantle (Magna et al., 2015), which has facilitated studies of the role of mantle versus crustal assimilation in generating shergottites (Figure 17), and also comparative planet accretion by comparison to the martian values to Earth, Moon and asteroid 4 Vesta. This work by Magna estimated the 7Li for Mars to be 4.2 +/- 0.9, within error of the estimate for Earth’s mantle 3.5 +/- 1.0 (Magna et al., 2015).
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